1. Field of the Invention
The present invention relates to a technique for preventing the electrostatic-surge oriented malfunction of a liquid crystal driving semiconductor chip which is to be mounted on a liquid crystal display panel (hereinafter referred to as “LCD”).
2. Description of the Related Art
An LCD is constructed by a segment-side glass plate on which a plurality of segment electrodes are formed in parallel in the vertical direction, for example, laying out a common-side glass plate, on which a plurality of common electrodes are formed in parallel in the horizontal direction, in such a way as to face the segment-side glass plate and filling a liquid crystal between the glass plates. The LCD performs display by using the property that as an electric field is applied between the segment electrode and common electrode, the direction of the liquid crystal between them is aligned to change the light transmissivity. As the segment electrodes and common electrodes should transmit light, they are formed of a material having both light transmissivity and electric conductivity in the form of thin films on the respective glass plates. A COG (Chip on Glass) type LCD has a liquid crystal driving IC (Integrated Circuit) chip mounted on a glass plate of a small LCD which is used for a watch, electric calculator or so.
FIG. 1 is a conceptual diagram of a COG type LCD.
This COG type LCD has an IC chip mounted on an extended segment-side glass plate of an LCD which has the segment-side glass plate and a common-side glass plate facing each other with a liquid crystal in between. Segment electrodes are extended to the electrodes of the IC chip by a segment wiring pattern formed of the same thin film material on the glass plate. Further, connector electrodes are formed on one side of the segment-side glass plate for connection to an external computer or so by a connector and wirings to connect the connector electrodes to the electrodes of the IC chip are also formed of the same thin film material as that of the segment electrodes on the glass plate by means of a lead wiring pattern.
FIGS. 2a and 2b are structural diagrams of a conventional liquid crystal driving IC chip to be used in the COG type LCD.
This liquid crystal driving IC chip 10 is to be mounted in the COG manner on, for example, the segment-side glass plate of an LCD. As apparent from the general structure in FIG. 2a, the IC chip 10 has a power-supply electrode 11 to connect to a connector electrode 1 formed on a segment-side glass plate, a plurality of address electrodes 12, a control electrode 13, a plurality of data electrodes 14, an enable electrode 15 and a ground electrode 16.
The power-supply electrode 11 is supplied with a power supply voltage VDD from an external computer or so. The address electrodes 12 are supplied with an address signal ADR from the computer for temporarily storage of display data. The control electrode 13 is supplied with a read/write control signal R/W from the computer. The data electrodes 14 are used to input and output a data signal DT to from the computer in parallel. The enable electrode 15 is supplied from the computer with an enable signal EN which indicates the enableness of the operation. The ground electrode 16 is connected to a reference potential for the computer, i.e., a ground potential GND.
The IC chip 10 further has a plurality of drive electrodes 17 for outputting a display drive voltage to the individual segment electrodes of a liquid crystal display section 2 and a plurality of drive electrodes 18 for outputting a scan drive voltage to scan the common electrodes of the liquid crystal display section 2 sequentially.
The address electrodes 12, the control electrode 13, the data electrodes 14 and the enable electrode 15 are connected to a control section 30, which controls the general operation of the IC chip 10, via a buffer 21, a buffer inverter 22, a bidirectional buffer 23 and a buffer inverter 24, respectively. Connected to the control section 30 is a RAM (Random Access Memory) 40 which stores display data. A display signal generating section 50 which generates display signals corresponding to the individual segment electrodes of the liquid crystal display section 2 is connected to the data output side of the RAM 40. Also connected to the control section 30 is a common signal generating section 60 which generates a common signal to scan the connector electrodes of the liquid crystal display section 2 sequentially.
The output side of the display signal generating section 50 is connected to the drive electrodes 17 via a plurality of drive sections 70S which generate display drive voltages, based on the display signals, to drive the respective segment electrodes in the AC manner. The output side of the common signal generating section 60 is connected to the drive electrodes 18 via a plurality of drive sections 70C which generate display drive voltages, based on the display signals, to drive the respective common electrodes in the AC manner.
Further, the IC chip 10 has a drive voltage generating section 80 which generates drive voltages V1 and V2 for AC-driving the liquid crystal display section 2 from a chip power supply voltage VDD-C supplied from the connector electrode 1. The drive voltages V1 and V2 are commonly supplied to the individual drive sections 70S and 70C.
The individual electrodes 11 to 16 of the IC chip 10 are connected to the connector electrode 1 via the lead wiring pattern formed on the segment-side glass plate as shown in FIG. 1. The individual electrodes 17 and 18 are connected to the liquid crystal display section 2 via the segment wiring pattern and a common wiring pattern both formed on the segment-side glass plate as shown in FIG. 1.
The drive section 70S comprises a predriver 71, four switches 72 to 75 and protective diodes 76 and 77, as exemplified in, for example, FIG. 2b. The predriver 71 outputs select signals SL1 to SL4 each for selecting an associated one of the four drive voltages VDD-C, V1, V2 and GND-C based on a display signal given from the display signal generating section 50 and a frame signal for AC-driving The switches 72 to 75 output drive voltages according to the select signals SL1 to SL4 and their output sides are connected to the corresponding drive electrodes 17. The protective diodes 76 and 77 serve to prevent the IC chip 10 from being damaged by the electrostatic surge that enter through the segment electrodes and common electrodes of the liquid crystal display section 2 and are connected between the drive electrode 17 and the power supply voltage VDD-C and the ground potential GND-C in the reverse directions with the normal operational voltage applied. The structure of the drive section 70C is the same as that of the drive section 70S.
The operation is discussed below.
First, as the power supply voltage is supplied to the power-supply electrode 11 and the ground electrode 16 of the liquid crystal driving IC chip 10 through the connector electrode 1, the power supply voltage VDD-C and the ground potential GND-C are supplied to the individual sections of the IC chip 10. Then, the drive voltage generating section 80 generates the drive voltages V1 and V2 and supply them to the respective drive sections 70S and 70C.
Data to be displayed on the liquid crystal display section 2 is given to the connector electrode 1 from an external computer. That is, the read/write control signal R/W to be given to the control electrode 13 is set to an “L” level which indicates writing. Then, the address signal ADR that designates the memory position in the RAM 40 is given to the associated address electrode 12 and the display signal DT to write data at the memory position is given to the associated data electrode 14. When the enable signal EN to be supplied to the enable electrode 15 is set to an “H” level under the situation, the display data is written at the designated address in the RAM 40. When the enable signal EN is “L”, the writing operation to the RAM 40 is inhibited.
The display data written in the RAM 40 is cyclically read out in order and supplied to the display signal generating section 50 under the control of the control section 30. The display signal generating section 50 generates display signals based on the display data read from the RAM 40 and supplies the display signals to the associated drive sections 70S.
In synchronism with the data reading from the RAM 40, the common signal generating section 60 generates a common signal to sequentially scan the common electrodes and supplies the signal to the drive sections 70C.
Accordingly, the drive sections 70C cyclically drive the common electrodes of the liquid crystal display section 2 in order, the display signal generating section 50 generates display information corresponding to the driven common electrodes and the drive sections 70S drive the respective segment electrodes. As a result, the liquid crystal display section 2 achieves matrix display according to the invention the display data stored in the RAM 40.
The IC chip 10 however has the following problem.
When a finger or so carrying static electricity touches the glass plate of the liquid crystal display section 2, for example, an electrostatic surge is applied to the segment electrodes or so via the glass plate. The applied electrostatic surge is transmitted to the drive electrodes 17 of the IC chip 10 through the segment wiring pattern on the top surface of the segment-side glass plate and then penetrates the drive sections 70S.
In case where the electrostatic surge has a negative polarity, the protective diode 77 in the drive section 70S is in the forward direction, so that the ground potential GND-C of the IC chip 10 is attracted toward the negative side. The ground potential GND-C is connected to the connector electrode 1 from the ground electrode 16 via the lead wiring pattern and is further connected to the ground potential GND of the external computer via the connector. Therefore, the negative electrostatic surge applied to the glass plate causes a surge current to flow to the finger or so from the ground potential GND of the external computer through the connector electrode 1, the lead wiring pattern on the segment-side glass plate, the ground electrode 16, the protective diode 77 and the segment wiring pattern.
As the lead wiring pattern on the segment-side glass plate, like the segment electrodes of the liquid crystal display section 2, is formed into a thin film pattern using a material which has both light transmissivity and electric conductivity, it has a relatively large resistance of about several hundred ohms. Therefore, the voltage drop caused by the surge current flowing to the lead wiring pattern makes the ground potential GND-C of the IC chip 10 lower than the ground potential GND of the external computer.
As the surge current does not flow to the lead wiring pattern that connects the enable electrode 15 to the connector electrode 1, on the other hand, the level of the enable signal EN of the enable electrode 15 is nearly the same as the level of the enable signal which is output from the external computer. In the IC chip 10, therefore, the level of the enable signal EN becomes relatively high as compared with the ground potential GND-C and may be determined as “H” although the level is “L”. While the operation is prohibited by the external computer, therefore, the IC chip 10 malfunctions to rewrite data in the RAM 40 so that the proper screen display cannot be accomplished.